Refrigeration systems for household refrigerators and freezers have heretofore been designed for low cost and high reliability, both of which require design simplicity together with a minimum number of parts. Typical refrigerators or freezers employ a vapor compression system having an electric motor driven hermetic compressor connected in a circuit with a condenser, evaporator, an optional accumulator, and a refrigerant flow restriction between the condenser and the evaporator.
The flow restriction is almost universally a capillary tube sized for optimal system efficiency under a nominal set of operating conditions. Such capillary tubes were designed for a constantly running refrigeration system operating at a single ambient temperature and constant load condition. Capillary tubes used as the sole restriction offered the advantages of low cost and high reliability. They performed satisfactorily under operating conditions other than those for which they were designed, albeit at reduced efficiency.
A system operating under these idealized design conditions utilized the condenser to liquify high pressure gaseous refrigerant from the compressor and delivered it, as a saturated or slightly subcooled liquid, to the capillary tube. The liquified refrigerant flowing through the capillary tube experienced a substantial pressure reduction on its way to the evaporator. Refrigerant was vaporized in the evaporator as it absorbed heat from a system load. The refrigerant then flowed to the compressor inlet as a low pressure gas.
When such a system operated under other than the optimum conditions it was far less efficient. For example, an extreme condition existed when the system "load" was light. In this case the heat in the refrigerated compartment was inadequate to evaporate the refrigerant in the evaporator so the evaporator tends to flood with liquid refrigerant.
This materially reduced the mass of refrigerant available in the system and consequently a mixture of hot gas and liquified refrigerant from the condenser tended to flow through the capillary tube into the evaporator. The gaseous refrigerant circulating in the system without condensing entered the evaporator and gave up heat to the liquified refrigerant there. The result was an undue burden on the compressor and significant system operating inefficiency.
When the load on the refrigerator or freezer was great and ambient atmospheric air temperature was high the system also operated inefficiently. In this condition the condenser could not reject sufficiently great amounts of heat to liquify all the refrigerant passing through it. Both liquified and gaseous refrigerant circulated in the system in these circumstances resulting in the operating problems noted above.
Although elimination, or at least reduction, of gaseous refrigerant flow into the evaporator was desirable to maximize efficiency, any significant restriction of hot gas flow at extremely high ambient temperatures was undesirable. Restricting such flow, for example by blocking communication from the condenser to the evaporator, was potentially damaging to the compressor.
In practice, the conditions under which household refrigerators or freezers operate vary widely from optimum design conditions. To accommodate varying conditions these appliances were constructed so that the compressor cycled on and off under control of a thermostat in the refrigerated compartment. When the thermostat was satisfied the compressor stopped. Refrigerant in the condenser continued to flow through the capillary to the evaporator until the system pressure equalized. This usually occurred after all the liquified refrigerant passed from the condenser into the evaporator.
When the thermostat restarted the compressor, gaseous refrigerant had to be compressed and recondensed for delivery to the evaporator before chilling could recur.
The rate at which the system pressure equalized and the rate at which chilling commenced again depended upon the degree of flow restriction created by the capillary. Capillary tubes sizes could be "loose" or "tight." i.e. less or more restrictive, respectively.
In a typical household freezer the capillary tube was sized "loose" to allow the evaporator to flood quickly during compressor start up. The "loose" capillary also allowed fast equalization of system pressure during the off cycle.
Fast evaporator flooding allowed the system to quickly reach a high running efficiency and reduced the compressor run time. Once the evaporator was flooded, however, this type of system tended to allow gas to enter the capillary tube and pass directly into the evaporator. As noted, circulation of hot gas in the system was inefficient and otherwise undesirable.
Furthermore, when the compressor turned off, the capillary tube continued to pass hot gas and liquid into the evaporator. This added more heat to the evaporator and further decreased overall system efficiency.
A principal advantage of a "loose" capillary design has been that fast pressure equalization enabled use of a low cost, low torque compressor motor for restarting the compressor after a short "off" cycle.
In typical household refrigerators "tight," or more restrictive, capillary tubes were used. Tight capillary systems tend to be slightly more efficient than "loose" systems during steady state run conditions. However when these systems were cycled on and off the "tight" capillary designs did not perform so well. The evaporators flooded so slowly during start up that any advantages in running efficiencies were lost over the entire cycle. Furthermore, pressure equalization took so long that low torque compressor motors experienced difficulty starting the compressor after a short off cycle. Such compressors were difficult to start against high back pressure.
In large refrigeration systems these problems were addressed by using a controlled expansion valve in place of the capillary tube. For example, Owens U.S. Pat. No. 3,367,130 discloses an expansion valve which opens and closes in response to the amount of subcooling of the refrigerant leaving the condenser by responding to a sensor attached to the external surface of the tube at that point. Valves of this type are too large and much too expensive to be substituted for a capillary tube in small household refrigeration systems.
Other proposals have involved using valves for blocking flow through the capillary tubes whenever the compressor turns off. These valves have been solenoid operated or have responded to changes in refrigerant pressure created by the compressor turning on and off. For example, see U.S. Pat. No. 4,267,702 issued May 19, 1981 to Houk. These kinds of valves did not modulate the refrigerant flows.
Still other proposals have suggested refrigerant flow modulating valves operated in response to liquified refrigerant temperature at the condenser outlet. These suggestions did not propose valve constructions capable of adequately controlling the flow of liquid refrigerant; did not provide easily manufactured structures; did not remedy problems caused by the circulation of hot gaseous refrigerant in the systems; and some did not block the refrigerant flow when the compressor was off.
The present invention provides a new and improved, highly efficient household refrigerator or freezer wherein a refrigerant flow controlling valve is provided which modulates the flow of liquified refrigerant through an expansion device in response to sensed condenser outlet refrigerant temperature and pressure conditions in a highly accurate fashion, blocks refrigerant flow from the condenser when the compressor is off and yet assures system refrigerant flow at extremely high ambient temperatures to protect the system.